Its not really supposed to have a popular mandate. The role of the courts is to establish primacy of law. If two legal standards conflict, individuals need to know which one has precedence. Otherwise, you end up with situations in which everything you do (or don't do) is functionally illegal.

When a Congress accumulates legislation over time that results in contradictory legal doctrine, it isn't the court's job to appease the public. Its the court's job to untangle the mess. Congress is then given the job of re-legislating to establish a legal code that has popular approval.

There are a lot of things wrong with the modern courts. But they are, at their heart, simply filling a bureaucratic role. They aren't supposed to be setting policy. They're supposed to be shaping policy such that it is a thing people can actually abide by.

It is more likely that recency bias is playing into it. I know a lot of libs are mad about the supreme court as of the last ~2 years and I'm sure things like them constantly hearing the same "well, he's not appointed by Biden so there's nothing he can do to remove him..." every time Louis DeJoy is brought up on stings a bit for some libs. Not to mention I feel like a lot of conservatives as of January 6th have been wary of being loud and proud about their fetish for the 'no tred!!' snake and their usual mask off fascism in general.